locking/csd-lock: Switch from sched_clock() to ktime_get_mono_fast_ns()

Currently, the CONFIG_CSD_LOCK_WAIT_DEBUG code uses sched_clock() to check
for excessive CSD-lock wait times.  This works, but does not guarantee
monotonic timestamps on x86 due to the sched_clock() function's use of
the rdtsc instruction, which does not guarantee ordering.  This means
that, given successive calls to sched_clock(), the second might return
an earlier time than the second, that is, time might seem to go backwards.
This can (and does!) result in false-positive CSD-lock wait complaints
claiming almost 2^64 nanoseconds of delay.

Therefore, switch from sched_clock() to ktime_get_mono_fast_ns(), which
does guarantee monotonic timestamps via the rdtsc_ordered() function,
which as the name implies, does guarantee ordered timestamps, at least
in the absence of calls from NMI handlers, which are not involved in
this code path.

Signed-off-by: Paul E. McKenney <[email protected]>
Reviewed-by: Rik van Riel <[email protected]>
Cc: Neeraj Upadhyay <[email protected]>
Cc: Leonardo Bras <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: "Peter Zijlstra (Intel)" <[email protected]>

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